1. Field of the Invention
Within the field of metrology, the invention contemplates the measurement of spherical and near-spherical optical surfaces with surface scanning based instruments.
2. Description of Related Art
The manufacture of precision optical surfaces requires high accuracy measurements of surface form. Generally, both reflective and refractive precision optical surfaces have a spherical form. However, many modern optical systems such as those used for micro-lithography applications include aspheric optical surfaces that depart somewhat from the spherical form to provide higher order optical manipulations.
A number of systems have been developed for measuring such near-spherical optical surfaces to required precision. These include both optical and non-optical systems. The optical systems typically use interferometric techniques, which include comparing a reference wavefront having the nominal shape of an optical test surface with a similarly shaped interrogating wavefront that is reflected from the optical test surface. The interrogating wavefront acquires characteristics of the optical test surface, which are revealed within interference patterns formed with the reference wavefront. Difficulties arise with accurately producing the two wavefronts and with arranging the interrogating wavefront to approach the optical test surface at normal incidence. The measurement of aspheric test surfaces using spherical reference and interrogating wavefronts is limited by issues of fringe density, resulting from a mismatch between the local shapes of the incident interrogating wavefront and the optical test surface. Such issues as aliasing and loss of contrast at a detector can result. Additional optical elements can be used to better match the reference and test wavefronts to the aspheric test surface. However, achieving the required accuracy for these null elements can be problematic. The accuracy is limited because the performance of the null elements is inferred rather than actually measured.
Two-wavelength interferometry has also been used to resolve ambiguities arising from high fringe densities. However, these techniques generally sacrifice resolution for range. Wavefront stitching can be used to measure larger areas by assembling limited zones of measurement over an optical test surface, for example, extended shapes such as hyper-hemispheres typically require the test surfaces to be illuminated from different positions to obtain partially overlapping interferograms of the surface, which must then be stitched together to form a map of the entire surface. These procedures are time consuming, computationally intensive, and subject to positioning errors.
Point-by-point profilometry is also used for measuring optical test surfaces by tracking probe displacements across the surfaces. Two problems compromise this approach. First, the fidelity of datum surfaces traced by the mechanical motions is subject to error. Second, probe performance degrades as the probe deviates from a normal orientation to the test surface.